78 



Hinge straight, of medium length, provided with a few 

 parallel elevated lines along its whole length, apparently 

 rounding somewhat into the postero-dorsal slope, which is 

 sharply carinate, and forms a regular declining curve pos- 

 teriorly. Antero-ventral side very oblike, arcuate, and not 

 carinate ; posterior extremity rather narrowly rounded be- 

 low. Surface ornamented by more or less distinct subim- 

 bricating concentric marks of growth. Length 1*40 inches ; 

 height (measuring at right angles across from the antero- 

 ventral side, to the posterior termination of the hinge) 0*65 

 inch ; breadth of the two valves 0*50 inch. 



If Mr. Koninck's genus Myalina is distinct from Mytelus, 

 which we think is the case, this shell must go in that genus, 

 as its hinge has exactly the characters of Myalina, as given 

 by Mr. Koninck. 



Locality — Same as last; it also occurs on the Missouri river, 

 opposite the northern boundary of Missouri. 



Bakevellia parva. 



Shell very small, subrhomboidal ; gibbous at the beaks 

 and down the oblique umbonial slopes ; antero-ventral mar- 

 gin sloping up very obliquely forward, and slightly sinuous 

 nearly under the beaks ; anterior extremity very nearly 

 rounded, or sub-angular; posterior side much broader, 

 rather narrowly rounded below, its upper margin sloping 

 first obliquely forward and upwards, then curving upwards, 

 or even slightly backwards, so as to meet the hinge line at 

 an angle of near ninety degrees. Postero-dorsal region 

 more or less compressed, and forming a kind of wing, which 

 is compressed so as to be more or less distinct from the gib- 

 bous flanks. Hinge straight, generally about equaling the 

 greatest length of the shell ; cardinal area narrow. Beaks 

 rather abtuse, oblique, and rising somewhat above the hinge 

 line, placed a little nearer the anterior end than the middle. 

 Surface and hinge unknown. Length 0*22 inch ; height 0*13 

 inch ; breadth about 0*09 inch. 



